Printing apparatuses such as a printer, copying apparatus, and facsimile apparatus print images of dot patterns on printing media such as a paper sheet and thin plastic plate on the basis of image information. Such printing apparatuses can be classified by the printing method into an inkjet type, wire dot type, thermal type, laser beam type, and the like. Of these methods, the inkjet method (inkjet printing apparatus) prints by discharging ink droplets from the orifices of a printhead onto a printing medium.
Recently, many printing apparatuses are used in various fields. High-speed printing, high resolution, high image quality, and low noise are required for these printing apparatuses. One of printing apparatuses which meet these requirements is an inkjet printing apparatus described above. The inkjet printing apparatus can perform noncontact printing by discharging ink from the printhead, and has an advantage capable of stably printing images on various printing media.
The inkjet printing apparatus is known to suffer various problems upon changes in environmental temperature and the temperature of a printhead integrating printing elements. This is because physical values such as the viscosity and surface tension of ink change depending on the temperature. In a so-called bubble-jet printing method of generating bubbles in ink by thermal energy and discharging ink by the generated bubbles, bubble generation conditions and the like also change upon a temperature change.
If the physical values of ink or bubble generation conditions change, the discharge amount of ink droplets from the inkjet printhead or the discharge position precision to a printing medium varies. This results in density variations, density unevenness, and a tint change in a printed image.
Hence, temperature detection control is important in the inkjet printing apparatus, and various control methods have been proposed for acquisition of the environmental temperature and head temperature. Examples of these proposals are as follows.
More specifically, an example is control of correcting an environmental temperature detected in accordance with the time elapsed after power-on of a printing apparatus (see, e.g., Japanese Patent Publication Laid-Open No. 5-31916, and U.S. Pat. No. 5,751,304). Another example is control in which means for measuring a time elapsed after previous printing and a temperature detection element for measuring the current temperature of a thermal head are adopted, and the temperatures of units except the thermal head are estimated using the current head temperature and the time elapsed after previous printing (see, e.g., Japanese Patent Publication Laid-Open No. 5-238045). Still another example is control in which printhead temperature detection means and a detection control step of detecting the printhead temperature after the end of printing every lapse of prospective time are provided, and the latest detected printhead temperature is regarded as an environmental temperature (see, e.g., Japanese Patent Publication Laid-Open No. 6-198886). Still another example is control in which a temperature detection circuit for detecting a temperature on the control board of a printhead and measurement means for measuring times elapsed after power-on of a printing apparatus, soft power-on, and printing are adopted, and the temperature read timing and detection temperature correction value are changed on the basis of the combination of the measured times (see, e.g., Japanese Patent Publication Laid-Open No. 7-60996, and U.S. Pat. No. 5,646,655).
Head temperature detection elements arranged on a printhead require detection temperature correction owing to manufacturing variations. As the correction method, there is proposed a control method in which head temperature detection means and environmental temperature detection means are adopted, and the offset value of a head detection temperature is set on the basis of the head temperature and environmental temperature upon powering on a printing apparatus or exchanging a printhead (see, e.g., Japanese Patent Publication Laid-Open No. 7-209031, and U.S. Pat. No. 5,646,655).
In this manner, in order to measure power-on time of a printing apparatus or a time elapsed after previous printing, conventional temperature control requires various time measurement means which always operate as long as the printing apparatus is connected to a power supply.
In recent years, reduction in the running cost of the apparatus and measures against environmental issues attract people's keen interest, and attention is given to power consumption upon soft power-off. Demands have arisen for stopping the time measurement means inside the printing apparatus main body.
A conventional desktop printing apparatus assumes to be always connected to the power supply, whereas in general, a portable printing apparatus is not always connected to the power supply when being carried. Thus, there is a need for environmental temperature acquisition control which does not require any time measurement means that always operates like a conventional one.
The printing apparatus exhibits large power consumption and a large heat generation amount in a printing operation in comparison with a non-printing state. To minimize the influence of heat generated in the printing operation, an environmental temperature detection element has conventionally been arranged at a portion almost free from the influence of a temperature rise in the apparatus. However, as the printing apparatus is downsized, the environmental temperature detection element tends to be influenced by a temperature rise in the apparatus regardless of the position of the element in the apparatus. This indicates that an accurate environmental temperature can be no longer acquired by a conventional method. As a result, the temperature detection means of the printhead cannot perform accurate correction.